In the field of orthopedic surgery, it is common to rejoin broken bones. The success of the surgical procedure often depends on the successful approximation of the bone and on the amount of compression achieved between the bone fragments. If the surgeon is unable to bring the bone fragments into close contact, a gap will exist between the bone fragments and the bone tissue will need to fill that gap before complete healing can take place. Furthermore, gaps between bone fragments that are too large allow motion to occur between the bone fragments, disrupting the healing tissue and thus slowing the healing process. Optimal healing requires that bone fragments be in close contact with each other, and for a compressive load to be applied and maintained between the fragments. Compressive strain between bone fragments has been found to accelerate the healing process in accordance with Wolf's Law.
Broken bones can be rejoined using screws, staples, plates, pins, intramedullary devices, and other devices known in the art. These devices are designed to assist the surgeon with reducing the fracture and creating a compressive load between the bone fragments. Screws are typically manufactured from either titanium or stainless steel alloys and may be lag-type or headless. Lag screws have a distal threaded region and an enlarged head. The head contacts the cortical bone surface and the action of the threaded region reduces the fracture and generates a compressive load. Headless screws typically have a threaded proximal region and a threaded distal region. A differential in thread pitch between the two regions generates compression across the fracture site. There also exists fully threaded headless compression screws that have a thread pitch differential over the length of the single continuous thread.
While these devices are designed to bring the bone fragments into close contact and to generate a compressive load between the bone fragments, these devices do not always succeed in accomplishing this objective. Among other things, the differential pitch on headless bone screws is able to reduce gaps and to initially create compressive loads; however, it is widely reported that the compressive load dissipates rapidly as the bone relaxes and remodels around the threads. Furthermore, with headless bone screws comprising two separated threads, the gap reduction is limited by relatively small pitch differential and short thread length.
Thus there exists a clinical need for fixation devices that are able to bring bone fragments into close proximity with each other, generate a compressive load, and maintain that compressive load for a prolonged period of time while healing occurs.